Sustained rolling of microparticles in shear flow over an electrostatically patchy surface.

This paper explores the particle-level dynamics involved in the capture of gently flowing microparticles on adhesive planar surfaces, governed by electrostatic interactions. The work focuses on conditions which produce sustained microparticle rolling, useful for the development of microfluidic devices which steer analyte particles and cells for manipulation and separation. In the regime where particle-surface interactions dominate particle-particle interactions, capture of individual negative silica microspheres, for thousands of microspheres, is studied on three model surfaces: negative silica, a flat polycation layer adsorbed on silica producing a strong positive charge, and an electrostatically patchy surface containing 6% areal coverage of flat 10 nm polycation coils. The patchy surface possesses a net negative charge close to that of bare silica. On the patchy surface, sustained rolling is observed for a substantial population of 1 microm silica particles, the ones which happened to diffuse close to the surface. Here, the velocity is near 2 microm/s (for a wall shear of 22 s(-1).) Run lengths for particle rolling exceed several hundred micrometers (usually exceeding the length of the microscopic field of view), with more particles escaping diffusively from the interface than permanently arresting. By contrast, firm particle arrest, with very few instances of rolling and a short run length when rolling did occur, was observed on the fully cationic surface. On the bare silica surface, a small rolling population was observed; however, the average run length was shorter than on the patchy surface. This study demonstrated how a patchy surface that produces adhesion through localized attractions can facilitate rolling in a shear field. The physicochemical heterogeneity acts like a surface roughness or a rapidly binding ligand-receptor pair, transferring stress and imparting torque across the interface.